There are known various forms of disk driving devices configured to record and/or reproduce information by rotating a magnetic recording medium in the form of a disk (hereinafter called a magnetic disk), for example. Among others, a disk driving device also called "hard disk device" is particularly used in a small-scaled, large-capacity system. Such a hard disk device is configured to rotate high revolution magnetic disks each made of a disk-shaped hard material having magnetic recording layers on surfaces thereof, and magnetic heads are opposed to the surfaces of the magnetic disks to record or reproduce signals thereon.
FIG. 8 shows one form of disk driving device of this type. The disk driving device generally comprises magnetic disks 1 on which information is recorded, magnetic heads 2 which record or reproduce information on or from the magnetic disks 1, a direct drive motor (not shown. Hereinafter called "DD motor") which drives the magnetic disks 1, a head driving mechanism 4 which moves the magnetic heads 2 to predetermined tracks on the magnetic disks 1, and a support board 5 which supports a housing which sealingly accepts therein the magnetic disks 1, the magnetic heads 2 and other members. The disk driving device further comprises a printed board 6 on which a motor driving circuit, control circuit, etc. are printed, and a frame (not shown) which holds the printed board 6 on the support board 5.
The illustrated magnetic disk device includes two magnetic disks 1. Each magnetic disk 1 has two recording surfaces on opposite planar surfaces thereof. Therefore, the illustrated disk mechanism includes four magnetic heads 1 associated with respective surfaces of the magnetic disks 1. The magnetic heads 2 are mounted on a swing arm 8 of the head driving mechanism 4 by canti-lever springs. The head driving mechanism 4 consists of the swing arm 8, a steel belt 9 mounted on a part of the swing arm 8, a pulley 10 on which an intermediate portion of the steel belt engages, and a stepping motor 11 which has a drive shaft 12 supporting the pulley 10 combined with the steel belt 9, so that when the stepping motor 11 is driven, the swing arm 8 swings about a pivot pin 8a thereof.
The magnetic disks 1, magnetic heads 2, swing arm 8, steel belt 9 and pulley 10 are accepted in the casing which consists of the support board 5 and a top cover (not shown). To establish an airtight sealing of the housing, gaskets are used at the contact between the support board 5 and the top cover and at the mounting portion of the stepping motor 11. Further, magnetic fluid is applied around the shaft of the DD motor for the same purpose. The swing arm 8 is provided with a shutter 17 extending outwardly away from the magnetic heads 2. Nearer to an airtight chamber of the support board 5 is provided a photo interrupter 18 serving as an outside sensor. The photo interrupter 18 defines an insertion path 18a which receives the shutter 17 loosely. In the prior art arrangement, when a magnetic head 2 reaches a zero track position at an outer circumference on an associated recording surface, the shutter 17 blocks the light path provided in the insertion path 18a of the photo interrupter 18.
In the arrangement using the stepping motor 11 to transport the magnetic heads 2, head positioning is difficult when the track density of the disks is increased. More specifically, since different materials in the hard disk apparatus have different expansion coefficients, there occurs a problem called "thermal off-track" in which the position of a magnetic head 2 relative to tracks on a recording surface varies with temperature. Therefore, in a 5.25 inch-type hard disk apparatus, it is difficult to precisely position the magnetic head 2 beyond 400TPI unless a servo system is used.
U.S. Pat. No. 122,503 discloses a control system using a servo system in which the inner-most and outer-most tracks are used as particular servo tracks. This system is called "ID-OD system" in abbreviation of "inner diameter" and "outer diameter". In this system, the disk apparatus is configured to first read the outer servo track and effect fine adjustment to place the head at the center of the track. Subsequently, the head is moved toward the inner servo track. In this operation, step pulses of the stepping motor in the head driving mechanism are counted, so that when the head reaches the inner servo track, the head positioning mechanism effects precise positioning to bring the head to the center of the track. While the accurate positioning is effected for each servo track, the positioning mechanism is informed of a correction amount necessary for finding the center of the track. Obtaining the correction amount, the positioning mechanism is enabled to calibrate precise positions of respective tracks according to information about the number of step pulses required for movement between the outer and inner tracks and about the fine step correction amount required in each servo track.
Further, the magnetic disk apparatus records information by saturation recording.
Saturation recording 2 is such that the current flowing to each head for its information writing is larger than a current value which saturates the magnetization of the magnetized layer of the magnetic disk in one direction. The saturation recording feature is in that new information can be written by "over-writing" which does not require erasure of old information. This simplifies the head construction and enables an instant changeover between reading and writing operations. Therefore, a single track may be divided into multiple sectors so that reading and writing may be effected per each sector, and this contributes to the maximum use of the recording surfaces without loss.
In order to write or read information on a magnetic disk, it is necessary to make a format in an information recording region on the magnetic disk. The format may be formed as shown in FIG. 9, for example, in which one cycle from first to fourth gap is divided into 32 sectors related to the exterior index signal EIN.
The first gap G1 is used to absorb a deviation of the exterior index signal EIN, and provided with "4E" written thereon at 16 byte unit. A sync field (VFO Sync Field) SF1 subsequent to the first gap G1 is used to lock the VFO (PLO) of the controller prior to an address search. Data digits are all zero, i.e. the data consists of clocks alone. In an ID field ID subsequent to the sync field are written check codes of an address mark, cylinder, head, sector and its region. A second gap G2 subsequent to the ID field ID is called "write space gap" and used for signal writing on a data field DF. The head is changed from its reading mode to writing mode therein with respect to the data field DF. Therefore, the second gap G2 provides the switching time. A sync field SF2 subsequent to the second gap G2 has the substantially same function as the first sync field SF1 preceding the ID field ID. However, the contents of the sync field SF2 are renewed concurrently with renewal of the subsequent data field DF which is used to write data thereon. A third gap G3 is provided subsequent to the data field DF. The third gap G3 is called "inner-record gap" and has a predetermined length because any change in the rotation of the DD motor 3 may destroy a subsequent sector S during a writing operation in a preceding sector S. The region from the leading sync field SF1 of the ID field ID to the third gap G3 forms one of 32 sectors, for example. A final, fourth gap G4 is provided subsequent to the third gap G3 at the 32nd sector from the first gap G1, and is provided with a predetermined signal to provide a length of the fourth gap G4 until detection of an exterior index signal EIN. The fourth gap G4 changes with velocity of the DD motor 3 and is called "speed tolerance gap".
The fourth gap G4 is 352 bytes in the most traditional aforegoing format of 256 bytes .times.32 sectors, and occupies about 4.1% of all the bytes. Practically, since the length of the byte changes with velocity of the DD motor 3, only a limited region near the first gap G1 is used for servo control seeking.
The rotating angle of a pulse generating magnet 40 attached to the rotor of the DD motor 3 is detected by a Hall element or other magnetic detection means, and used as the first interior index signal IN1. The exterior index signal EIN corresponding to the first gap G1 is supplied when a predetermined number is counted after detection of the first interior index signal IN1. That is, the exterior index signal EIN is applied to the host computer 26 when the aforegoing count number is detected after detection of the interior index signal IN1, and the position represents the beginning of the recording tracks T.
Such a format is usually formed in the recording regions except those having servo information thereon before shipment of the system, and a formatting is effected by an end user when he first uses the magnetic disk driving device having the magnetic disk therein to enable information writing in the data field of the format.
In an ID-OD or other system in which servo information is provided in limited tracks alone, positional control of the head is effected based on the servo information which is written in a relationship with exciting phases of a stepping motor or other device for transporting the magnetic head. Therefore, if the stepping motor mis-steps for any reason, it sometimes occurs that the formatting is written, erasing the servo information. If the formatting is once written on the servo information, the system cannot obtain the servo information thereafter, and cannot position the head accurately.
Further, disk driving devices known heretofore are configured to detect the zero track of a magnetic disk, using a sensor or other mechanical means, or alternatively using a particular signal specifically written in the radial direction for zero track detection to unable rewriting in the region having the particular signal.
However, the use of such a mechanical means invites an increase of the manufacturing cost of the system because of an additional cost not only for the sensor or other means itself but also for a more difficult assembling process caused by more complicated zero track positional adjustment. The use of the particular signal to unable rewriting thereon necessarily requires a slow-down of the disk rotation to adjust the transporting speed, and this causes an instablity of the head assembly and an increase of the error rate.